The present invention relates to a test stand for motor vehicles with a roller assembly comprising a contact surface for rolling the wheels, with the contact surface being rotatably driven and adapted to be blocked, and/or braked, and/or released.
Test stands of the generic type have been known from practical operation for a long time. They are used in motor vehicle repair shops and also by the TÜV [German Technical Inspection Association] within the scope of the periodic performance test of motor vehicles. Normally, the operability or effectiveness of the brakes is checked.
In the art known from practical operation, there basically exist three different methods of testing automotive brakes. Until now, it has been common practice to test with very special equipment, such as test plates, test rollers, or by means of a decelerometer. Testing with test plates and by means of a decelerometer is known as a dynamic test. The use of test rollers relates to a static test. The difference between the dynamic test and the static test consists in that the test with test plates or by means of a decelerometer makes it necessary to move the vehicle. The test plates and the decelerometer are stationarily arranged. However, in the case of the test by means of test rollers, the vehicle wheel and the test rollers rotate. The foregoing description explains the main difference between the two basic testing methods.
In a plate brake tester, the test plate is used to simulate a portion of the road, with the plate being supported on rollers. The vehicle drives over the test plate, which connects in the travel direction via a measuring element to the surroundings, for example, a drive-on ramp. When the vehicle is braked on the test plate, the braking forces developing during the braking are picked up via the measuring element. The measuring procedure will last only as long as the vehicle remains on the test plate which has a limited surface. In practice, the measuring time ranges from 0.5 to 1 second.
The faster the vehicle is driven onto the test plate, the greater are the recorded braking forces. A braking force that is to be recorded at most can, however, not be higher than the friction between the tires and the test plate. Otherwise, the slip limit will be exceeded. In the case of the plate brake tester, it is, however, disadvantageous that the measuring result is dependent on the drive-on speed. Moreover, the measuring can be performed only over a very short duration, since the length of the brake plate is limited. Last but not least, the plate brake tester is not very practical, since it is extremely difficult for a layperson to start the braking maneuver exactly at the point, i.e., exactly upon driving onto the test plate.
Because of its constructional design, the roller brake tester is nothing more than an endlessly long road, which is simulated by rotating rollers. This makes it possible to test the braking operation over any length of time.
The mode of operation of the roller brake tester results from its constructional design. Over a tester frame, the vehicle drives slowly onto the roller assembly, normally two rollers, until it comes to a stop on the test rollers. The test rollers are driven by an electric motor, in most cases via chains. The electric motor is usually supported for oscillation. When the wheel is braked, the electric motor will have to apply more force to rotate the wheel. This force is measured via a sensor, which is usually constructed as a torque support.
The roller brake tester known from practice is a static system, in which the vehicle is stationary. The rollers can be rotated for any length of time, so as to permit testing the brake system of the vehicle in almost any state. Contrary to the plate brake tester, the test is by no means limited in time, and any driving condition, depending on the drive of the test rollers, can be repeated as many times as desired. However, similarly to the plate brake tester, it is not possible to test the brake beyond the coefficient of adhesion of the roller surface, since the slip limit is then exceeded.
Further, a conventional roller brake tester has the disadvantage that the tires directly rotate on two rollers and are impressed or flexed by the rollers. This falsifies the test result because of different static frictions of the particular situations, i.e., as a function of weight, so that the roller brake tester appears to be disadvantageous in this respect.
It is further known from practice to test the braking action of a vehicle by means of decelerometers. A decelerometer ultimately operates with an accelerated mass or weight, which is mounted on rollers and connected via a spring to a frame. During the measurement, the decelerometer lies horizontally in the direction of the movement. During the braking operation, the weight, in this instance the motor vehicle, is deflected in the travel direction. The displacement is measured and converted into deceleration. To this end, it is also possible to use an acceleration sensor, which operates by the same principle. As the braking deceleration sets in, the same forces are active as the own weight of the test body itself.
In practice, the decelerometer is problematic, since it permits determining only the total deceleration of the vehicle, but not the distribution of the braking force over the individual wheels. Consequently, this type of testing is applied only in exceptional cases, i.e., generally only when a vehicle cannot be tested by the two foregoing methods because of technical conditions.
Last but not least, it is common to perform in the test stands of the art, the functional tests of vehicle components with respectively one tester that is suitable for the particular test. In this connection, it is preferred to use rotating drives or brakes for determining the braking force and performance. To determine track values, one uses either track testing plates or wheel alignment analyzers. Shock absorbers are tested with shock absorber testing devices, and joint play is tested via pneumatically or hydraulically operated test plates. Testing of the individual parameters is sequential, in that the vehicle moves from testing device to testing device, or depending on need, that the testing device moves to the vehicle. Only in few exceptions is a combination device offered, which connects, however, via test rollers to the wheel. This is not road-conforming and considerably falsifies the actual wheel contact forces as they occur on the road.
A test stand is disclosed, for example, in U.S. Pat. No. 1,957,455, wherein the wheel is supported on a driving belt which assumes an arcurate path. While this increases the wheel contact surface, it does not correspond to the condition on the road.
It is therefore an object of the present invention to provide a test stand of the initially described type, which permits realizing a particularly indicative performance test with constructionally simple means.